1. Field of the Invention
The present invention relates generally bidirectional, electromagnetic communications systems, and more particularly to bidirectional electromagnetic communication systems adapted for communication through a partially conductive medium.
2. Description of the Related Art
Magnetic loop communications systems have found niche applications in several areas. In one method of implementing inductive communications a primary coil generates inductive, quasi-static and propagating fields, which induce a voltage in a remote secondary coil. This signal is processed to extract electronic data as a means of implementing a communication link. In these systems the same mechanism is used for the return path of a bidirectional link. And both ends of the link have identical hardware and equal power requirements. This is referred to as a “direct” inductive link.
Some loop antenna communications systems are based on the mutual inductance effect such as Radio Frequency Identification (RFID) systems, and the like. In these systems the primary coil generates a magnetic field that induces a current in a remote secondary coil. This secondary coil current generates a secondary magnetic field, which opposes that of the primary coil according to Lenz's law. Variation of a load impedance across the secondary coil varies the current allowed to flow and hence varies the secondary magnetic flux generated. The concept of mutual inductance describes the effect on a primary coil's impedance due to a coupled secondary coil. Magnetic coupling is weak in the link described here but the effect is still one of mutual inductance. The impedance of the primary coil has some dependence on the current flowing in the secondary. Opposing magnetic flux generated in the secondary reduce the self-generated magnetic field at the primary, which acts to oppose flow of current. In this way the current in the primary is modulated by load variation across the secondary coil and a communication mechanism is established.
RFID systems based on mutual inductance are single directional. The main benefit of these systems is the low power requirement at the data source. The primary coil can be considered as an exciter source since it provides power for the system but does not transmit data. The secondary coil transmits data by modulating its load. Passive RFID tags can be deployed which have no internal power requirements so are not limited by battery life. This class of system is a “mutually inductive” communications link.
There is a need for improved bidirectional communication systems. There is a further need for bidirectional communication systems that have a direct inductive communications downlink and a mutually inductive uplink.